1. Field of the Invention
The present invention is related to an organic electroluminescence display and a method for manufacturing an electroluminescence display.
2. Description of the Related Art
An organic EL (Electro Luminescence) element emits light when a current is supplied to a light emitting medium layer between two opposed electrodes, and, in order to achieve efficient light emission, it is important to keep a film thickness of the light emitting medium layer to about 100 nm. Further, in the case of forming a display from the organic EL element, it is necessary to perform patterning on the organic EL element with high definition.
A low molecular material and a high molecular material can be used as the organic light emitting material for the light emitting layer formed on a substrate or the like. The low molecular material is subjected to resistive heating vapor deposition and the like to form a thin film, and at the same time the patterning is performed by using a microscopically patterned mask. However, this method has a problem in that patterning accuracy is reduced with an increase in size of a substrate.
Therefore, the high molecular material has recently been used as the organic light emitting material, and a method of forming a thin film by wet coating with a coating liquid obtained by dispersing or dissolving the organic light emitting material into a solvent has been tried. Spin coating, bar coating, projection coating, dip coating, and the like are known as the wet coating method for the thin film formation. However, the wet coating methods have difficulty in realizing high definition patterning and color coding with R, G, and B, and it is considered that the thin film is most effectively formed by a printing process that is capable of achieving excellent color coding and patterning.
A glass substrate is often used as a substrate in an organic electroluminescence element or a display. Thus, a method which uses a metal hard printing plate such as a gravure printing method is unsuitable. Offset printing which uses a rubber blanket having elasticity and a relief printing method which uses a rubber printing plate and photosensitive resin printing plate having elasticity are suitable. A method (Japanese Patent Laid-Open No. 2001-93668 Official Gazette) using offset printing and a method (Japanese Patent Laid-Open No. 2001-155858 Official Gazette) using relief printing are actually proposed as an attempt to achieve these printing methods.
In addition, a cylindrically pressing type relief printing machine comprises a cylinder type blanket cylinder and a flat surface table (not illustrated in the figure) which is fixed and placed at a predetermined position. Specifically, this machine includes the following components: a flat surface table for horizontally fixing a flat relief printing plate at a predetermined position; a flat surface table for horizontally fixing a substrate (a substrate to be printed) at a predetermined position; an ink supplying roller which attaches an ink on a top surface by moving, rotating and touching on a surface of a relief printing plate placed and fixed on a surface table for fixing a relief printing plate; and a blanket cylinder, wherein an ink on the top surface is transferred to a blanket which has a surface is made of rubber, by rotation and movement of the blanket rotating on a surface of a relief printing plate in a stand by mode of an ink supplying roller, and further an ink transferred on a surface of a blanket is transferred to a substrate (a substrate to be printed) by the rotation and movement of a blanket cylinder, thereby printing is performed.
It is known that an ink of a viscous type (in other words, thixotropy type) or a liquid type used for a relief printing method has the most suitable viscosity and surface tension. Especially, a viscosity modifier such as a thickener or a surface activating agent for adjusting a surface tension is generally added to a liquid type ink.
In the case where electronic materials are printed, solubility thereof may be limited or impurity may be not preferably included therein.
Especially, when a film is formed by printing an organic light emitting material using a printing method, an organic light emitting material is dispersed or dissolved in a solvent such as water, alcohol or an organic solvent (including a binder resin if necessary), thereby an ink for printing or coating is made.
It is generally known that in a case where a pattern of a film of an organic light emitting material is formed and the pattern is made to emit light in a element, the higher the purity of a film made of an organic light emitting material is, the better the durability of an element is. Therefore, since the thickener or the like remaining in the organic light emitting material lowers purity, the thickener or the like can not be included in an ink. Therefore, from this reason, the scope adjustable properties of an organic light emitting material ink for obtaining an ink transfer property of a printed matter and achieving stability of a pattern shape is limited.
Due to the above-mentioned reason and low solubility, especially in the case of a light emitting material case, only some kinds of aromatic solvents can be used, thereby reducing the range of usable inks.
A display panel for use in mobile terminals such as a mobile phone, PDA (personal digital assistant) needs a high definition display of 100 ppi or more. In such a high definition display, the distance between pixels is short, for example, 40-10 μm. Therefore, in the case where positional accuracy during printing is poor, a printing pattern may be shifted near to an adjacent pixel and solidify. In addition, even if position accuracy during printing is not poor, in the case where a liquid printing ink is near a printed pattern of an adjacent pixel, a solidified printed pattern may be dissolved again in an approaching printing ink, and color mixture frequently occurs.
Especially, in the case where a material having a long light emitting wavelength (roughly, long (red)>(green)>(blue)short) is mixed in a material having a short light emitting wavelength, a phenomenon of energy transfer causes a material having a long light emitting wavelength to be preferentially emitted in an organic EL element. That is, in the case where blue having a short wavelength is mixed with red having a long wavelength, the color of the light which is emitted shifts significantly from blue, and the color of the light emitted is close to white.
In addition, formation of a light emitting layer by a printing method is significantly influenced by the wettability of a surface of a region to be formed. Therefore, in the case where a layer directly below (hereinafter, an underlayer) of a light emitting layer is conventionally formed only inside a pixel of a partition wall substrate, wettability of an upper surface of a partition wall which is not printed is different from wettability inside a pixel. Therefore, a film thickness of a light emitting layer can not be stably uniform.
Further, because a light emitting layer is formed on an underlayer, a film forming state of the light emitting layer changes depending on the film forming state of the underlayer. That is, if a film thickness of an underlayer is not uniform, concave and convex of a surface or non-uniform wettability of a surface produce a light emitting layer of having a non-uniform film thickness and thereby, uneven light emitting occurs.
The present invention provides an organic electroluminescence display and a method for manufacturing the display, the display having the following merits. Shift of chromaticity minimized as much as possible, a light emitting layer having a uniform film thickness is formed and thereby, a uniform and stable light is emitted and production yield is improved.    [Patent Document 1] JP-A-2001-93668    [Patent Document 2] JP-A-2001-155858    [Patent Document 3] JP-A-2001-155858